1. Field of the Invention
The invention relates to a self-adaptive equalizer for use in the receiver of a data modem for correcting in the baseband the data signal received. The equalizer comprises a difference circuit to which is applied the received signal and a correction signal formed at a signal sampling frequency by a transversal filter having adjustable coefficients, the difference circuit producing a corrected signal which is applied to a decision circuit for recovering the data signal. The recovered data signal is applied to the input of the transversal filter, the coefficients of which are adjusted so as to minimize a predetermined function of an error signal derived from the corrected signal.
2. Description of the Prior Art
Obviously, equalizers acting on a baseband data signal find direct usage in modems receiving data transmitted in the baseband. But it is alternatively possible to use equalizers of this type in modems receiving data transmitted by carrier modulation, by having the equalizer act on the baseband signals derived in the receiver by means of demodulation of the received signal.
With respect to the received signal, the equalizer under consideration behaves as a recursive filter whose loop comprises the cascade arrangement of a decision circuit and a transversal filter. In this type of equalizers, commonly known as "Decision Feedback Equalizers", the transversal filter operates, at sampling instants, on the signal supplied by the decision circuit and its coefficients are adjusted in a manner such that it generates at these sampling instants a correction signal which is a copy of the interference signal resulting from the data transmitted preceding the data then present at the input of the receiver. This correction signal is applied to the difference circuit where it is subtracted from the signal received so as to form a corrected signal from which the interference has been removed. The criterion used for adjusting the coefficients of the transversal filter is minimize a predetermined function (generally the meansquare value) of an error signal which is characteristic of the interference signal component of the corrected signal produced by the difference circuit.
In prior art equalizers (see, for example, the article "An Adaptive Decision Feedback Equalizer", published in IEEE Transactions, Vol. COM-19, No. 3, June 1971, pages 281-292), the error signal is formed at each sampling instant as the difference between the corrected signal produced by the difference circuit and the fixed-level data signal recovered by the decision circuit. But the error signal thus formed does not only depend on the interference signal produced by the preceding data signal and still present in the corrected signal until the equalizer has converged, but also on the level of the actual datum also present in the corrected signal as this is the datum to be recovered by the decision circuit. This level may vary as a function of the length of the link and/or the transmission conditions. To ensure that the error signal is characteristic of the interference signal, it is necessary to stabilize in the known equalizers the data level in the signal applied to the equalizer, with the aid of an automatic gain control (AGC) arrangement.